A while ago, My Movies came out with an iPhone application that lets you sync the meta data of your movie collection to your phone. This lets you see which movies that you own while you are out, so you can know whether you should rent or buy a movie. This synchronization happens by the PC application syncing the collection to the My Movies online collection, and then your phone syncs from there.
They just released an Android application that has the same functionality. This is yet another benefit of using My Movies to manage my movie collection.